1. Field of the Invention
The present invention relates to a cryptographic processing apparatus and a cryptographic processing method, and a computer program, more particularly to a cryptographic processing apparatus and a cryptographic processing method, and a computer program, which improve robustness against a differential analysis known as a decryption process or an attack process.
2. Description of the Related Art
In these days, as network communications and electronic commerce have been developed, there is an important issue in ensuring security in communications. One method for ensuring security is an encrypting technology. In fact, communications using a variety of encrypting methods are carried out at present.
For example, a system is put into practical use where an encryption processing module is embedded in a small apparatus, such as an IC card, data transmission and reception are carried out between the IC card and a reader/writer as a data read/write apparatus, and an authentication process and encryption/decryption of transmission/reception data are carried out.
There are various encryption processing algorithms, which can roughly be classified into a public key cryptosystem where an encrypting key and a decryption key are set up as different keys, for example, a public key and a secret key, and a common key cryptosystem where an encrypting key and a decryption key are set up as common keys.
There are also various algorithms in the common key cryptosystem. One of them is a system where a plurality of keys are generated based on a common key and it repeats and performs a data conversion process on a block-by-block basis (64 bits, 128 bits, etc.) using the thus generated plurality of keys. A typical system of the algorithms employing such a key generation system and a data conversion process is a common key block cryptosystem.
An algorithm of the typical common key block encryption may be, for example, a DES (Data Encryption Standard) algorithm as U.S. standard encryption, which is used widely in various fields.
The algorithms for the common key block encryption represented by tire DES are mainly divided into a round function section which performs conversion of input data, and a key schedule section which generates a key applied at each round of the round function (F function) section. The round key (subkey) applied at each round of the round function section is inputted into the key schedule section and generated based on one master key (main key), and applied in each round function section.
However, in such common key encryption process, there is a drawback that the key and the encryption algorithms may be leaked by way of decryption. As a typical method of the decryption or an attack method, the differential analysis (also referred to as the differential decryption system or a differential attack) is known, which analyzes an application key in each round function by analyzing a majority of input data (plain text) and their output data (cipher text) which have certain differences.
Ease of key analysis by way of decryption means that the security of the encryption processing is low. In a conventional DES algorithm, a process (conversion matrix) applied in a linear conversion section of the round function (F function) section is the same at the round of each step, which makes it easy to perform the differential analysis. As a result, the key analysis can be performed without much difficulty, thereby posing the drawback in the application of such encryption process.